In today's RF systems, signals received by antennas usually have high frequencies and narrow channel bandwidths. To filter out a desired channel directly, a high Q filter is required. For reasons regarding gain, precision, stability, and power consumption etc. of circuits for processing high frequency signals, it is not practical to demodulate a signal at a high frequency range directly. In a commonly used approach, a signal is first down-converted to an intermediate frequency (IF) range. The down-converted signal is then filtered, amplified, and demodulated to filter out the desired channel, but other problems are introduced.
In a low IF method, there may be image frequency interference. If a desired signal's frequency and an interference signal's frequency are symmetrical about a local oscillator signal's frequency, the desired signal and the interference signal will be down-converted to the same intermediate frequency. The interference signal is an image frequency interference of the desired signal.
A zero IF method solves the problem of image frequency interference. However, there may be problems of DC offset, 1/f noise, and signal leakage, etc.
Hooman Darabi proposed a SAW-less method for filtering a blocker signal (“A Blocker Filtering Technique for SAW-Less Wireless Receivers”, IEEE JSSC, Vol. 42, No. 12, December 2007, pp. 2766-2773). In this method, a received signal is down-converted, and then a blocker signal is separated from the down-converted signal. After that, the blocker signal is up-converted, and the up-converted blocker signal is subtracted from the received signal to obtain a desired signal. In this method, the main signal path and the translational path share the same local oscillator signal in the down-conversions. Although the method solves the problem of blocker signal partly, other problems arise. Therefore, it is necessary to provide a new method and a new apparatus to solve the problems mentioned above.